THE STEM AND THE LEAF 67 
The hard-wood trees show great differences in the rate at 
which their trunks increase in thickness. Poplars, basswoods, 
willows, and red oaks, growing in good soil and unshaded, may 
for forty or fifty years form annual rings as much as three 
eighths of an inch thick, but old beeches 
and sugar maples in the forest, after they 
have passed the hundred-year limit, often 
grow not more than about one sixteenth 
of an inch per year. When very old, 
though still sound, they may grow only 
about one twenty-fifth of an inch per year. 
Would it be good policy to let beeches 
and maples remain long in the forest 
after they are one hundred years old 
before cutting for timber? Why ? 
65. Growing points. The extreme tip 
of the live stem or root of a dicotyle- 
don consists of a more or less conical or 
cushion-shaped mass of tissue composed 
of thin-walled cells like those of the cam- 
bium layer. This portion of the stem or 
root is called the growing point. Every 
live twig and rootlet is tipped with a 
growing point, and it is by the rapid 
sub-division and consequent multiplica- 
tion of these cells that the lengthening 
of the main stem and its divisions, and 
of every root, takes place. 
All branches originate from growing 
points, which are usually developed along 
Fic. 46. Formation of 
knots due to branches. 
The figure gives part of 
a lengthwise section of 
a stick of birch wood 
a, section of the base of 
a branch which persisted 
until the tree was felled ; 
b, section of the base of a 
branch which died some 
years earlier and is now 
covered by several layers 
of younger wood 
leaf-bearing portions of the stem, each one just above the point 
where a leaf is attached. In their earliest beginnings both 
leaves and rudimentary branches consist wholly of thin-walled 
cellular tissue. Fibrovascular bundles, connected with those 
of the underlying stem, soon appear in the branches and leaves 
as their development goes on. 
